PROJECTSUMMARY Sleep disturbances predict risk of Alzheimer?s disease (AD). Sleep-wake cycles critically regulate brain interstitial fluid (ISF) levels of A? and tau, two critical proteins that accumulate in AD. Both A? and tau are released by neuronal activity, which is higher during wakefulness than in sleep. Moreover, sleep is a critical phase during which factors in the ISF are cleared from the brain. Therefore, sleep disturbances affect daily function and also contribute to disease progression. However, little is known about which brain regions are affected inAD to give rise to sleep disturbances,making itdifficult to identifythe circuit level mechanismsthat drive dysfunction, or to design targeted therapeutic strategies. This project tests the hypothesis that the thalamic reticular nucleus (TRN) is a criticalbrain region in AD,and that impairments in itsactivitydrive sleep disturbances and exacerbate disease progression. The TRN is a major component of the thalamocortical- corticothalamic network that regulates sleep, attention, and memory, whichare all affected in AD. However, little is known about the state of TRN in AD patients or in animal models. We found that in transgenic mice expressing mutant human amyloid precursor protein (APP mice), TRN activity is strikingly reduced, in the absence of cell loss. Such reductions in TRN activity led to sleep fragmentation and reductions in slow wave sleep(SWS),andpredictedthemagnitudeofA?depositioninbothhippocampusandcortex,whichmayrelate tothefactthatSWSisthephaseofsleepduringwhichactivity-dependentproductionofA?isreduced,andA? is cleared from the brain. Moreover, deficits in SWS and sleep maintenance manifest early in disease in APP mice, prior to hippocampal deficits, suggesting that TRN impairment may both predict and contribute to disease progression. The goals of this proposal are to identify cellular mechanisms that impair TRN activity, and test if selectively manipulating neuronal activity in the TRN cannormalize sleep, reduceA? accumulation, and improve memory. To achieve these goals, in Aim 1 we will use electrophysiology and pharmacology in thalamic slicesto identifythe intrinsic, synaptic, andnetworkproperties of TRN that result in its hypoactivity in APP mice. In Aim2, we willuse DREADDs to acutely activate TRN cells in APP mice totest if TRN activation affects dynamics of interstitial A?, and/or memory consolidation. In Aim 3, we will use DREADD-mediated activation ofTRN in APP mice to test if chronic activationof TRN cannormalize sleepparameters, reduceA? accumulation,andimprovememory.Resultsfromthisprojectwillhavemajorimpactbecausethey:1)highlight a vulnerablenetworkearly in diseasethat maypredict and contributeto diseaseprogression, and2) identifya novel therapeutic strategy with potential to normalize sleep, improve memory, and delay disease progression in Alzheimer?s disease. Insights gained will also be used to derive general principles about the dynamics of AD-relatedproteinslikeA?andtauinthebrain,whichwillimpactourabilitytotreatthiscomplexdisease.